At the time of press working, a stamping force by a press machine, a reaction force of the material to be worked deformation reaction and the like act on a tool and the tool elastically deforms. Such elastic deformation is called a strain of the tool.
FIG. 25 shows a conceptual view of the tool strain occurring at the time of press-forming in a press machine constituted of a punch 2, a die 7 and a blank holder 4. The solid line shows the outer shape of the tool before press-forming, and the dotted line shows the outer shape of the tool when the tool elastically deforms at the time of press-forming. FIG. 25 shows the deformation with emphasis, but the elastic deformation amount in the load range of actual forming is in the order of about several micrometers.
FIG. 25 shows only the deformation of the punch 2, the die 7 and the blank holder 4, but to be exact, it is conceivable that the elastic deformation also occurs to the other press mechanism elements such as a press machine slider, and a guide pin. However, the dominant elastic deformation in a press forming phenomenon is considered to be the deformation of the punch, die and blank holder, and the elastic deformation relating to three of the punch, die and blank holder will be discussed as the strain of the tool hereinafter.
Occurrence of a tool strain reduces the dimensional accuracy of a formed product. The deformation amount and deformation distribution of the formed product due to a tool strain change in accordance with the stamping force by the press machine, reaction force by the material to be worked deformation resistance and the like. Therefore, the tool strain changes due to change of the various conditions such as the press machine, tool shape, quality of the material to be worked, shape of the material to be worked, lubrication and stamping force, and the change of the tool strain causes quality scatter between the stamp parts. In the forming prediction by the finite element method or the like cannot take the tool strain into consideration due to the calculation ability and the like, and therefore, the tool strain makes the prediction of forming by the finite element method difficult.
As the device for controlling a tool strain, Patent Document 1 discloses a device for correcting half-releasing for a press brake in a press brake which bends a workpiece between a punch and a die by operating the punch mounted to an upper beam and the die mounted to a lower beam to contact and separate from each other, and the device including a plurality of strain sensors for the upper beam which are provided along the longitudinal direction of the above described upper beam and detects only the strain of the above described upper beam, a plurality of strain sensors for the lower beam which are provided along the longitudinal direction of the above described lower beam and detects the strain of the above described lower beam, a plurality of actuators which are disposed to spread between the above described lower beam and the lower tool, or between the above described upper beam and the upper tool, along the direction of the bending line, and apply stamping force in the vertical direction to the above described lower tool or upper tool, and a control means that stops descend of the above described upper beam partway before completion of pressing after start of the pressing, fetches detection outputs of the above described strain sensor for the upper beam and the above described strain sensor for the lower beam at the time of stopping state, calculates strain amounts of the upper beam and the lower beam based on the respective detection outputs, controls drive of the above described plurality of actuators so that the strain amounts of the upper beam and the lower beam become the proper values based on the calculated values, and thereafter conducts control of restarting pressing control. Thereby, the formed product having a uniform bending angle over the entire length is to be obtained.
Patent Document 2 discloses a press tool in a tool press forming characterized by including a load detection means, a stroke detection means, a detection means of press frequency, detection means of tool temperature, a deformation prediction model constituted of a single model or a plurality models of an abrasion model of the tool, a thermal deformation model of the tool, a load deformation model of the tool, a thermal deformation model of a material to be worked and a spring back model of the material to be worked, a multivariable control signal generator and a drive device which deforms the internal wall of forming recessed part. Thereby, the product having dimension and shape with high accuracy is to be obtained.
Patent Document 3 discloses a press-forming device which does not control a tool strain, but is characterized by having a punch, a die and a blank holder, an abrasion force measuring means mounted between the above described die and the above described blank holder, and a blank holding force regulating means. Thereby, a proper frictional force can be applied without recourse to the variation factor such as lubricity between the tool and the workpiece and surface property, and a favorable formed product is to be always provided regardless of the variation of the material characteristics and environmental change.
Patent Document 1 discloses the invention relating to the device having the function of measuring a tool strain, but it does not disclose the invention except that the strain sensor for the beam is provided along the longitudinal direction of the beam for the press brake. Therefore, in order to conduct quality control with high accuracy in press-forming using a tool having a shape more complicated than the beam for press brake, the invention of Patent Document 1 cannot sufficiently measure a tool strain occurring in the tool having the complicated shape, and the invention of Patent Document 1 is not sufficient.
Further, Patent Document 1 discloses the invention relating to a device controlling a tool strain, but while the strain detection parts used for detection of a strain of the upper and lower beams for the press brake are installed at the upper and lower beams, the actuator used for strain control of the upper and lower beams is installed between the lower beam and the lower tool, or between the upper beam and the upper tool, and the strain detection position and the strain control position differ.
Accordingly, when the invention of Patent Document 1 is applied to the tool having the shape more complicated than a tool for a press brake, such as a draw forming tool, strain control by the actuator exerts an influence on not only the strain amount at the strain amount detection position which is desired to be controlled, but also on the strain amount at the strain amount detection position which is not desired to be controlled, and therefore, the S/N ratio as control becomes low. Further, in forming with the tool having a complicated shape, the contact pressure distribution acting on the tool is not uniform, and the strain amount distribution occurring to the tool is complicated. Accordingly, the desired strain control amount differs according to the strain amount detection position. Therefore, in the constitution of the invention of Patent Document 1, the actuator control for controlling the strain control amount to the desired amount is difficult.
Further, in the invention of Patent Document 1, forming is temporarily stopped during forming, the strain amounts of the upper and lower beams are detected in the stopping state, the control by the actuator is conducted so that the strain amounts of the upper and lower beams become proper values, and thereafter, forming is restarted. However, unlike forming mainly constituted of bending as the press brake, in draw forming, the frictional force between the material to be worked and the tool significantly differs from the frictional force during forming when forming is intermitted halfway. Therefore, when the invention of Patent Document 1 is applied to draw forming, the measured tool strain amount differs from the tool strain amount during forming, and control accuracy becomes worse.
Further, in the invention of Patent Document 1, working has to be temporarily stopped during forming, and the cycling time of forming becomes worse by carrying out the control according to the invention of Patent Document 1.
Patent Document 2 discloses the invention relating to the device controlling a tool strain. The invention uses the deformation prediction model which predicts the deformation states of the tool and the material to be worked based on the reduction in thickness detected by the stroke detection means, the load detected by the load detecting means and the temperature detected by the detecting means of the tool temperature, and estimates the correction amount of the forming recessed part shape required for obtaining the product of a predetermined dimension and shape from the prediction result to perform control. The deformation state of the tool is the prediction using the model, and is not directly measured.
Patent Document 3 discloses the following invention as the principle of directly measuring the frictional force. Namely, the flat plate and the blank holder are fastened with a bolt or the like to sandwich a strain measuring element, and when a workpiece is sandwiched by the die and the above described flat plate and slid in this state, a shearing strain occurs to the above described strain measuring element and the frictional force can be measured. This intends to measure the frictional force by installing some structure in the blank holder or the die, but does not directly measure the tool strain of the blank holder or the die.
In order to conduct quality control with high accuracy, it is indispensable to measure the tool strains of the punch, die and blank holder directly, and for this purpose, the inventions of Patent Documents 1 to 3 are insufficient.
Thus, the present invention has an object to provide a press-forming device and a press-forming method which is capable of controlling a tool strain during press work and has high accuracy and high applicability. The present invention particularly relates to a press-forming device and a press-forming method which measure a tool strain occurring during press work.    [Patent Document 1] Japanese Patent Application Laid-open No. Hei 5-337554    [Patent Document 2] Japanese Patent Application Laid-open No. Hei 9-29358    [Patent Document 3] Japanese Patent Application Laid-open No. 2004-249365